Lysine and Lysine polymers. Lysine is a versatile, trifunctional amino acid. At physiological pH, lysine has a charge of plus one, having one anionic carboxylate group and two cationic primary amine groups. It is relatively inexpensive and available throughout the world for use mainly as a nitrogen supplement for feeding livestock.
Lysine is produced commercially, via a combination of chemical and enzymatic routes, by several companies. The principal form of commercially provided lysine is the zwitterionic salt with chloride, i.e. lysine HCl. In this form, the carboxylate group of lysine is neutralized as a counterion to one amino group of another lysine molecule, with the other amino group occurring as the chloride salt. Lysine sulfate is also commercially available. In this case, the carboxylic group of the lysine molecule occurs in the acid form, with both amino groups counterionic to sulfate.
Lysine may also be obtained as the zwitterionic free base (α carboxylate as COO−, ε amine as NH3+, α amine as NH2). This form is also available in small amounts as a monohydrated form, lysine hydrate.
The polylysine structure below shows two residues bonded via an alpha linkage (peptide or amide bond) between the α-amino group of one monomer and the α-carboxylic group of another.

Polylysine is used in biochemical and biomedical research to facilitate cellular attachment; see e.g. Meyers and Beck 1985; Zamora et al. 2005; Campbell et al. 2006. The large body of biomedical literature about polylysine also includes many citations of its utility as a component in drug delivery formulations and controlled release systems (e.g., Carpenter et al. 2009; Farokhzad et al. 2009, Harmon et al. 2009, Meers et al. 2009).
Polylysine has also been proposed for use in a diversity of other applications. For example, it has been shown in earlier studies as potentially superior to monomeric lysine as a feed supplement for livestock (Bercovici et al. 1987); more recently it has been contemplated for use in oil well drilling in thickening compositions for sealing well bores (Eoff et al. 2009, 2010) and for promoting separation of oil from drilling fluids and solids, along with hypochlorite (Reddy 2007).
Access to high molecular weight polylysines, however, has been limited. The microbially-produced product is a poly-ε-lysine of about 30 residues, thus having a molecular weight of about 3,870. The method of production also imposes extra costs as well as practical limitations on commercial quantities that are made.
Thermal polymerization of the zwitterionic free base form of lysine has been used to produce lysine homopolymers and lysine copolymers with other amino acids (Strojny and White, 1965). The products of these experiments exhibited insecticidal, antimicrobial, and fungicidal activities. However, molecular weights (degree of polymerization ˜10) and yields were low. Lysine in this form has significant vapor pressure and sublimates to a large extent when heated. Such low molecular weight polymers would be ineffective as coagulants due to their relatively low binding affinity.
Lysine may be polymerized via chemical means, such as the reaction of N-carboxy anhydrides in the presence of initiators such as sodium methylate in solvents (Bercovici et al., 1987). This route has been used for preparing polylysine molecules of varying molecular sizes for research purposes. These are commercially available, generally in small quantities (e.g. from Sigma-Aldrich). However, because the starting N-carboxy anhydrides are generally prepared by phosgenation of the amino acid, this route is problematic from a commercial point of view.
Also described in the prior art are thermally polymerized copolymers, based principally on aspartic acid or related anionic monomers, including maleic acid, which incorporate lysine as a minor component (e.g. Koskan et al. 1999; Wood and Calton 1996a, 1996b; Machida et al. 2001). These copolymers are polyanionic, not polycationic, and they were designed for uses such as dispersants in detergents, antiscalants in boiler water, water-absorbing resins, and other uses unrelated to the field of the present invention.
Coagulants and Clarification of Water Produced in Oilfield Operations. Water is used in oil recovery operations for a variety of reasons. For example, in steam-assisted gravity-drainage (SAGD) operations, it may be injected as superheated steam into the geologic formation to facilitate liquefaction of oily deposits at depth, followed by movement of the pressurized, oily aqueous stream to the surface. Here, the oil is separated from the water. Typical ratios of water to oil in such operations range around 2:1 to 3:1; that is, there often results approximately 2 to 3 barrels of water containing oily residuals per 1 barrel of oil that is recovered.
In principle, the oil may be separated from the water via straightforward techniques, such as flotation and skimming, that take advantage of the differences in the densities of oil and water. However, in practice, the leftover water stream, even after skimming, decanting, and preferential draining, is often dark and oily, owing to the presence of stable emulsions and suspensions of oil and oily solids in the water.
These components are thought to be ionically dispersed in the water as oily micelles having outwardly facing anionic groups, mainly carboxylates, along with some sulfated and phosphated aliphatic, cyclic, and heterocyclic hydrocarbon moieties of complex and heterogeneous composition. In addition to the oily droplets that are thus stably emulsified in the water column, there can be significant components of mineral residuals such as micron-scale particles of sand and clays. Total suspended solids and oils in the range of 1% by weight or more can render the water unusable without further clarification and separation steps.
Ideally, in zero discharge approaches to oil recovery such as SAGD or cyclic steam stimulation (CSS), the water needs to be clarified to acceptable levels so that it can be recycled to the steam generators without fouling and clogging downstream process equipment like organic removal filters and ion exchangers. In larger-scale, open-mining operations, the practice in general is to recycle the clarified and cleansed water to the process, with an eventual goal to return it to some extent to the environment, typically a river, from which it originated.
As exemplified in SAGD and CSS operations, the oil is recovered from an aqueous process stream that is brought to the surface from the depths of the formation, still superheated and under pressure. The process stream is slowed down into very large vessels where the bulk of the oil and water naturally and largely separate, primarily based on their densities. At this stage, chemical additives, such as de-emulsifiers, are typically employed to promote both rate and degree of separation. The oil is removed and sent to pipelines and tankers for transport to upgraders and refineries for further processing.
This removal of the bulk oil leaves behind a stable reverse emulsion of residual oil and oily solids in water, termed produced water. This produced water is cooled via heat exchangers to a temperature in the range of 90° C. so that it can be handled more readily in the downstream process steps that include addition of water clarification chemicals.
The preferred chemical treatment at this step typically has involved injection of a polymeric, polycationic coagulant to disrupt the anionic dispersion and to thus promote coalescence of the oily droplets and solids into small particulates. At this point, the produced water stream so treated moves at high velocity, for example around 8 feet per second in pipes, but it quickly flows next into large skim tanks with low velocity and high residence times (typically several hours). The oil and oily solids that float are removed by skimming. At this point, the water stream has typically cooled to a temperature of about 80° C.
The next step in the process involves an increased rate of flow of the partially clarified produced water into induced flotation devices, where microbubbles are introduced, along with a flocculant. The bubbles are intended to stick to the oily flocculated particulates and make them float, allowing them to be decanted so that the produced water stream can be further clarified. After this process, the stream is directed through organic removal filters so that any residual potential foulants will not reach and interfere with the final steps of silicate removal and ion exchange. The water is then recycled to the steam generator, prior to re-injection into the deep well.
The ion exchange membranes and devices, and the boilers as well, do not tolerate oil and oily residuals and can be easily compromised, if not ruined, by an input of improperly de-oiled water, either suddenly or gradually. This results in downtime, which is very costly in terms of lost production, and may require replacement of ion exchangers as well as other components of the de-oiling line.
In conventional practice, treatment of the oily produced water stream is quite commonly ineffective, resulting in only partially clarified water. Even if the initial treatment does result in coalescence of oily solids and clear partitioning of oily and aqueous phases, the subsequent attempts at removal of residual oil and oily solids is often inefficient.
In addition, even if the water is clarified and properly separated from oily solids, residual treatment chemicals may remain as soluble components of the clarified process stream. Typically, these materials are vinyl polymers that are mainly comprised of acrylamide and quaternized residues. Consequently, they are nondegradable, environmentally persistent, and sometimes considered undesirable from the standpoints of possible harmful biological effects.
To address these problems, the present inventor previously developed a class of biodegradable flocculants that compete well with vinyl flocculants on both cost and performance. See, for example, PCT Pubn. Nos. WO 2007/047481 and WO 2010/011867. These flocculants have been used in combination with conventional coagulants in protocols that quickly and effectively de-oil produced waters, generating a clear aqueous phase and a separated phase of oily solids.
However, heretofore, there have been no biodegradable coagulants for use in treatment of produced water that meet cost/performance requirements. Coagulants typically are polycationic. Some polycations that are based on biological feedstocks are available; these include cationized starches, chitosan, and tannin amines. However, these types of polycations do not perform adequately as coagulants, are too expensive, or both.